Referring to FIG. 1, a conventional display device includes a substrate 11, such as a printed circuit board or a thin-film transistor (TFT) substrate, and a plurality of pixel units 100. Each of the pixel units 100 includes a red light-emitting diode 12R, a green light-emitting diode 12G and a blue light-emitting diode 12B formed on the substrate 11 for respectively emitting red, green and blue light. FIG. 2 illustrates the formation of the pixel units 100 on the substrate 11. A temporary sapphire substrate 201 is utilized to allow a specific type of light-emitting diodes, such as red light-emitting diodes 12R, green light-emitting diodes 12G and blue light-emitting diodes 12B, to be formed thereon. Then, a multiple-transfer method was adopted to transfer the light-emitting diodes 12R, 12G, 12B onto the substrate 11, which is correspondingly formed with a plurality of solder pads 203. The light-emitting diodes 12R, 12G, 12B can be transferred onto the substrate 11 by pressing the temporary sapphire substrate 201 toward the substrate 11 with the light-emitting diodes 12R, 12G, 12B correspondingly adhered to the solder pads 203. Subsequently, the temporary sapphire substrate 201 can be removed using, for instance, laser lift-off techniques. However, since different kinds of light-emitting diodes may have different heights as shown in FIG. 2, pressure exerted on the substrate 11 due to the pressing action may be uneven and may result in unequal bonding strengths between the light-emitting diodes 12R, 12G, 12B and the substrate 11. Moreover, since the dimensions of the light-emitting diodes 12R, 12G, 12B have been gradually reduced with advancement of technology, precise alignment between the light-emitting diodes 12R, 12G, 12B and the solder pads 203 may become relatively difficult.